On Tue, 3 Mar 2009, Jon Harrop wrote:

> Functors give
> you the same capability in OCaml but they are rarely used precisely because
> the functionality is not very useful.

I think I disagree with this.  I think functors aren't used very much in 
Ocaml because:
1) They're a big, scary name, and
2) They're slightly less efficient.

The biggest difference between Haskell and Ocaml that I see is simply the 
difference between attitudes of the two communities.  The Ocaml community 
is like "Don't use functors- they disable inlining and cost you six whole 
clock cycles on a function call!  They're evil, I tell you!"  Meanwhile, 
the Haskell community is like "I used typeclasses all over my application, 
and the performance didn't completely suck- woot!  Type classes rule!" 
This is a broad generalization, and not completely accurate- but on the 
whole, the ocaml community is much more focused on (clock cycle) 
efficiency, while the Haskell community is much more focused on 
abstraction and programmer-cycle efficiency.

The type classes comparison isn't even an analogy- it's a precise 
relationship.  Anywhere you might be thinking, in Ocaml, "this would be a 
nice place to use a type class", use a functor.  You want operator 
overloading in Ocaml?  You got it: use a functor.  If this causes you a 
knee jerk reaction about performance, ask yourself this: do you know how 
type classes are implemented in Haskell, and what their performance hit 
there is?  Now, imagine programming haskell where typeclasses are only 
used in a very places- Ord, Eq, Monad.  No Num.  No Monoid.  No Show. 
That's Ocaml.  Not that it has to be.

Having actually used Haskell for a while, I think I actually like functors 
better than type classes.  But that's a rant for a different venue.  The 
big difference is that Haskell programmers use type classes, and the Ocaml 
programmers don't use Functors (very often, if at all).

Brian